The Grey to Green project makes the most of the new planted areas as landscapes to manage the rain water off the road.
Sheffield’s development, like any city, has by necessity involved sealing over surfaces with concrete and tarmac and of course the buildings themselves and ‘efficiently ‘ collecting and disposing of rainwater through gulleys, gratings and pipes typically sending it to the treatment works or nearby river.
The natural processes that would have happened before this urbanisation involve the rainfall being caught and held by vegetation, soils and the hollows of the landscape.
This natural spongy nature of the land means streams and rivers also behave more naturally. Cities, however, increase the rate and volume of flow that end up in these watercourses potentially causing flooding. Where flows are directed to sewers there is potential at times of heavy rain for sewage polluted overflows entering the river. The run-off also picks up substances such as oils and poisonous metals from surfaces, often from vehicles, that if untreated enter the watercourses potentially affecting wildlife.
Grey to green tries to mimic these natural processes by allowing rain run-off to trickle over the edge of the length of the road into a new dished landscape known as a swale, in this case made of special soils and filled with colourful plants. Whilst there will be some water that will soak deeper into the ground, the concrete low walls crossing the swale allow water to be held within each area with below and above ground features that control the flow from the higher to lower areas. In times of extreme rainfall flow potentially will overtop the walls in a cascade.
The everyday rain run-off that effectively washes the road will be cleaned of its pollutant in the stone, underlying soils and planting. This scheme is effectively reconnecting the highway surface to its natural catchment, the River Don, by redirecting good quality water to the river whilst removing it from the previous route to the treatment works that was via the gulleys and sewer.
Increasing intensity of rainfall associated with climate change is likely to increase the potential for flooding and sewer overflows. Schemes such as Grey to green provide examples of how to improve the resilience of our city to these effects.